1. Field of the Invention
The invention concerns a metal gasket or joint whose original design provides it with an extremely high unit-area pressure for a relatively low gripping power.
2. Discussion of the Background
Having regard to their insensitivity to corrosive fluid, their sound behaviour at high and low temperatures and their long-time strength, metal gaskets are used in a large number of applications in, for example, the chemical, petroleum, nuclear, space and automobile fields, this list being in no way restrictive.
The quality of the imperviousness procured by a gasket depends in particular on the unit-area pressure P developed between the contact surfaces of the gasket and the flanges of the assembly in which this gasket is placed. During the initial tightening of the assembly, the unit-area pressure needs to be sufficient so as to allow for adaptation of the gasket to the surface roughness of the flanges. Moreover, when the this assembly is in service, in installation, the following equation R must always be satisfied: EQU P.sub.sp &gt;m.times.p
where m represents the gripping coefficient of the gasket, namely the ratio (always greater than 1) between the minimum gripping pressure of the gasket and the pressure of the internal fluid when the installation is in service, and where P represents the pressure of the internal fluid when the installation is in service.
Thus, it is clear that the value of the unit-area pressure P.sub.sp needs to be as high as possible so that the assembly remains sealed to high internal pressure P.
A further significant characteristic of the gaskets concerns the gripping power required to be applied to the assembly so as to obtain imperviousness. In a large number of applications, it is essential that this force remains relatively slight. This is particularly the case when the assembly is not very accessible and renders difficult the handling of gripping implements (in the nuclear industry, for example) and when the assemblings with materials presenting sophisticated characteristics need to be lightened and do not support high forces (in the aerial navigation or space industries, for example).
In the prior art, the metal gaskets, which are designed so as to be put in place with a relatively slight gripping power, still comprise in the parts in contact with the surfaces opposite the flanges a relatively thick outer ductile metal casing or covering. When a relatively slight gripping power is applied, this metal deforms beyond its limit of elasticity and ensures imperviousness of the assembly. Some of the metal gaskets having this configuration include solid joints with a K, V or U-shaped section, as well as Helicoflex joints (registered trade mark), which are toric-shaped joints comprising an elastic metal core constituted by a spring with contiguous spires and one or more metal casings, the outer casing being made of a ductile metal.
However, the presence of a ductile metallic casing in the parts of these joints in contact with the surfaces of the flanges has the effect of significantly increasing the contact surfaces of the gasket and the flanges at the time the gripping power is applied. The unit-area pressure developed between these surfaces thus remains relatively reduced. Accordingly, having regard to the equation mentioned above which must satisfy the unit-area pressure so as to ensure satisfactory imperviousness of the assembly in service, this type of gasket may not be used when the pressure of the internal fluid is too high.
Furthermore, the mechanical properties of the ductile metals constituting the outer casing of these existing gaskets are altered when the temperature becomes too high.
This state of the technique is illustrated particularly by the document FR-A-2 557 662 which concerns a Helicoflex gasket (registered trademark) whose outer casing made of a ductile material comprises a projecting part with a triangular section which disappears completely when the gripping power is applied to the assembly.
This examination of the state of the technique shows that there currently exists no metal gasket for which imperviousness may be obtained by applying a relatively slight gripping power (about 10 daN/mm, for example) and which needs to be suitable so as to guarantee imperviousness of the assembly at high pressure and/or at high temperatures.